I find it embarrassing that it needs to be explained why this line is not grammatically correct and doesn't make sense, but since some of "top" commie editors believe the line is perfectly fine, I feel like I should explain.
Just to clarify, not being a native speaker, I felt like I should have more than just my own understanding of the language to back up my reasoning, so I consulted other editors (native speakers), who confirmed what I said and expanded on that.
I am perfectly familiar with the phrase "the extent of which." Nothing wrong with that in itself. However, that phrase introduces a dependent clause. Let's look at how it works.
"The tornado caused massive damage, the extent of which is still being assessed." Here "the extent of which" expands on the "damage."
"Exploding a bomb here could cause a blackout, the extent of which would be hard to predict." Again, a dependent clause relating to the blackout.
You can look at "the extent of which" as a substitute for "whose extent," which would be more suitable for persons, but those usually don't have an extent. Let's try the substitution.
"The tornado caused massive damage, whose extent is still being assessed."
"Exploding a bomb here could cause a blackout, whose extent would be hard to predict."
Disregarding the issue of using "whose" with abstract concepts, these lines flow just fine.
Now let's look at the word "but." This word joins two independent clauses.
An independent clause can stand by itself. It makes sense by itself.
A dependent clause doesn't make sense on its own. For example:
"The extent of which is still being assessed." Nope, that's not a meaningful sentence.
"The extent of which would be hard to predict." Nope, that doesn't make sense by itself either.
Let's try that supposedly independent clause, introduced by "but," from our line:
"The extent of which is out of my hands." Oh hey, this doesn't make any more sense than the previous two.
To make things a little more confusing, a certain commie editor told me that "it's meaningless by itself," and "it makes no sense until you put it with the other sentence," but after I said that "an independent clause makes sense by itself," he replied, "it does make sense [...] it just refers to the previous line."
Now, I'm not really sure what his verdict is on whether the line does or does not make sense, but if it only makes sense by referring to the previous line, then it can't be an independent clause.
Look at this:
1. "It makes no sense until you put it with the other sentence."
2. "It makes sense by itself."
Can you see a difference between these two statements? I'm sure you can. The first one pretty much describes a dependent clause, while the second one describes an independent clause.
Now, let's turn our line into something with actual independent clauses.
I'll use an example somebody suggested in our discussion:
"I could go easy on someone, but the extent to which I do so is out of my hands."
Interestingly enough, when I consulted a competent editor, I got this:
"I could go easy on someone, but the extent to which I do so would be out of my hands."
The latter is better, since it follows "could" in the first clause with "would be" rather than "is," but it's pretty much the same.
To that our commie editor replied: "That would make it pointlessly long."
No, it wouldn't be pointless. It would turn the line into something that makes sense. But at least I think he agrees that this would be correct.
(Also note that they used "the extent to which," not "the extent of which." One means "to what extent," the other means "whose extent," as I explained earlier. The original editor, however, insisted that "of" is correct here.)
"The extent to which I do so would be out of my hands."
This is a sentence that makes sense on its own, as opposed to: "The extent of which is out of my hands."
In this latter case, the word "which" has no reference point.
Let's check a bigger segment from the script, just so we're sure what the meaning was supposed to be.
"My Energy Drain is an irreplaceable purrt of who I am,
and not a skill I can fureely use, let alone control.
I could go easy on someone, but the extent of which is out of my hands.
It's pawsible that I might kill someone without even intending to."
So what is it exactly that's out of her hands? She can try going easy on someone, but whether that will be enough to prevent killing them is out of her hands.
Here are some other examples people have come up with:
"I can go easy on people, but it's out of my hands as to how far."
"I could go easy on someone, but how easy (I go on them) is out of my hands."
"I can try to go easy on someone, but even that is not always enough."
"I can try to go easy on someone, but whether that'd be enough is out of my hands."
Even some of that is still unclear, though.
The thing is, it's not about the extent of how easy she goes, but the extent of how effective that will be.
Or as my friend put it:
"As it is, it seems like it's the extent of *her going easy on them / how easy she goes on them*, but it's not."
If you think about what the "which" really refers to, it's something like:
"the extent of *the effectiveness of my going easy on them*," so to quote my friend again:
"That 'which' isn't even IMPLICIT in the line as is."
So in the end the line is even more unclear than it seemed at first glance.
We tried to create a similar sentence to the one in the screenshot, and came up with some examples:
"I was trying to listen to John, but the voice of whom was quiet and unclear."
Here, "the voice of whom," being the same as "whose voice" would have to follow the word "John" to make sense. Put a "but" in there, and you get this monstrosity.
"I can try to get as much done as possible, but the quality of which will probably be suspect."
The antecedent of "which" there is some unclear thing that's barely implicit in the preceding clause.
I was thinking about how to use "the extent of which" in the line and make it correct, if at all possible. The best I can come up with is something like this:
"I could go easy on someone with efficiency, the extent of which is out of my hands."
It's a pretty damn terrible sentence to say, but at least "the extent of which" clearly refers to the efficiency, and the sentence makes sense grammatically.
This is how you use "the extent of which," preferably directly after what the "which" refers to.
(It may sound a bit less awkward as: "I could go easy on someone with efficiency whose extent is out of my hands.")
Anyway, once you stick a "but" in there, you change the second part to an independent clause, in which you can't use "the extent of which" anymore because it makes no sense.
So one issue here is that we're mixing grammatical structures that can't work together.
Another issue is that we're confusing "the extent to which" (to what extent) and "the extent of which" (whose extent).
And another issue is that what this "which" refers to is not even a part of the sentence at all (the extent of *the effectiveness of my going easy on them*).
And yet, some commie editors who "actually know how english works" will tell you that "there's nothing wrong with that line."